For joining mutually workpieces made from a synthetic thermoplastic resin material, a method for laser welding is known.
Such the laser welding is executed for example as follows. As shown in FIG. 1, a workpiece made from a laser-transmissible material and another workpiece made from a laser-absorptive material are put together. Laser is irradiated from the laser-transmissible workpiece side towards the laser-absorptive workpiece side. The laser transmitted through the laser-transmissible workpiece, is absorbed into the laser-absorptive workpiece and causes the exothermic. The laser-absorptive workpiece at a laser-absorbing neighborhood is melted by the exothermic. Then it melts the laser-transmissible workpiece, to weld both workpieces. After cooling, the laser-transmissible workpiece and the laser-absorptive workpiece are firmly joined with sufficient welding strength.
The following merits of the laser welding are mentioned. They are feasibility of the welding without contact of a laser beam source and the workpieces being due to weld; little thermal influence of periphery by the local exothermic; no threat of mechanical vibration; feasibility of the welding of the precise workpieces or the three-dimensional or complicated workpieces for themselves; superior reproducibility; maintaining of sufficient airtightness; great welding strength; difficulty of recognizing a boundary line of the welding by visual observation; no generation of dust and so on.
The laser welding is executed by simple operation for joining certainly. The laser welding achieves equal or more joining strength, as compared with prior joining of resin workpieces for example clamping by clamps such as bolts, screws and clips, bonding by a bonding agent, vibration welding, ultrasonic welding. Also it achieves labor saving, improving of productivity, and decreasing of working cost, because of little thermal or vibratory influence. So the laser welding is applied to joining of functional workpieces or electric workpieces that is due to avoid the thermal or vibratory influence under an automobile industry and an electric or electronic industry. And the laser welding is adapted to joining of resin workpieces having an intricate shape.
As a prior art of the laser welding in Japanese Patent Provisional Publication No. 11-170371, a method for the laser welding is mentioned. The method comprises a step that an opaque workpiece made from a laser-absorptive synthetic thermoplastic resin and a colorless transparent workpiece made from a laser-transmissible synthetic thermoplastic resin are put together, and then the laser is irradiated in focus at a put portion thereof together. When the welded portion is watched from a side of the colorless transparent workpiece, the welded portion and a non-welded portion differ in color and smoothness and look unattractive.
In Japanese Patent Provisional Publication No. 2000-309694, anthraquinone-type dyes as colorant for a laser-transmissible colored thermoplastic resin composition are mentioned. If the composition is preserved for a long-term, the colorant would be sublimed. Then color tone thereof fades out from the workpiece made from the composition. Therefore the welded position looks unattractive.
It is desired that a laser-transmissible colored thermoplastic resin composition does not cause fading out a hue under a thermal treatment procedure before the laser welding, does not cause sublimation of colorant in the composition, and has sufficient laser-transmissible property. Furthermore it is important that the colorant in the laser-transmissible colored thermoplastic resin composition has sufficient heat resistance and fastness, because the prepared laser-welded article must not sublime the colorant and must not fade the color tone out under stark exposure atmosphere such as high temperature and high humidity.